The present invention relates generally to temperature regulation and more particularly to an apparatus and method for regulating the temperature in a cryogenic test chamber.
Apparatus for subjecting a test specimen to cryogenic temperatures typically includes an enclosed, thermally-insulated vessel into which a fluid, generally a liquid at a cryogenic temperature, is introduced. For example, liquid helium having a temperature of about 4.2 degrees Kelvin is frequently used for this purpose. The specimen is placed in a test vessel which in turn is located in a thermally-insulated test chamber inside the cryogenic vessel. Some of the liquid is drawn from the cryogenic vessel into the test chamber through a passageway such as a coupling tube by partially evacuating the test chamber to create a pressure differential between the interior of the test chamber and the interior of the remainder of the cryogenic vessel. As the liquid enters the test chamber it cools the test chamber, and the specimen enclosed in the test vessel located therein, to the cryogenic temperature of the liquid.
The apparatus is operated in a high temperature mode if it is desired to subject the specimen to a temperature higher than that of the liquid. A heater located in 10 the test chamber is energized, boiling the liquid and heating the resulting gas until the desired temperature has been reached. A thermostatic control can be used to maintain any desired temperature higher than the temperature of the liquid for as long a time as desired.
A low temperature mode of operation is utilized if it is desired to subject the specimen to a temperature lower than that of the liquid. The test chamber is evacuated until the pressure therein is low enough to cause the liquid to boil at the desired temperature. By selecting a suitable pressure and continuing to evacuate the test chamber as the liquid boils so as to maintain that pressure, any desired temperature down to about 1.5 degrees Kelvin can be reached. Such a temperature can be maintained until all the liquid in the test chamber has boiled away, and then more liquid must be admitted to the chamber and the process repeated.
The coupling tube through which the liquid is admitted to the test chamber must be large enough in diameter to assure a sufficiently high liquid mass flow rate into the test chamber to cool the chamber to the temperature of the liquid in a reasonably short time. However, when the apparatus is being operated in the low temperature mode, the pressure differential which results from the continuous evacuation of the chamber tends to draw more liquid into the chamber through the coupling tube, and the larger the tube the faster the liquid is drawn in. This is undesirable because the newly-admitted liquid tends to raise the temperature in the chamber, thereby making it more difficult to attain the desired lower temperature. This problem can be solved, at least in theory, by providing a mechanical valve to close the coupling tube during operation in the low temperature mode, but the difficulty of constructing reliable low temperature valves has limited the usefulness of this approach.
A different problem is encountered when operating in the high temperature mode, especially when trying to attain a temperature between about 4.2 and 20 degrees Kelvin. Energizing the heater boils some of the liquid in the test chamber, but absorption by the boiling liquid of latent heat of vaporization ("latent heat of vaporization" is heat which is absorbed by the liquid as it changes to its gaseous phase) tends to cool the chamber, thereby offsetting the warming effect of the heater. Thus, the temperature in the chamber tends to remain at about 4.2 degrees until all the liquid has been changed into its gaseous phase, at which time there is a sudden jump in the temperature to about 20 degrees due to the effects of the heater, which now are concentrated entirely on the gas in the test chamber. If the heater is shut off in an effort to cool the chamber back down to the desired temperature, the gas gradually cools, but as it cools its pressure drops, tending to draw more liquid into the chamber through the coupling tube. This accelerates the cooling process until the temperature drops below the desired temperature. Energizing the heater causes the cycle to repeat. Thus, the system behaves as a relaxation oscillator, the temperature oscillating back and forth between 4.2 and 20 degrees rather than stabilizing at the desired temperature.
In addition to these problems, the presence of liquid at cryogenic temperature in the test chamber during the high temperature mode of operation can interfere with insertion and removal of samples and cause errors in certain types of measurements, difficulties which would be avoided if there were no liquid in the chamber.
It will be apparent from the foregoing that there is a need for a way to achieve reliable temperature regulation in a cryogenic test chamber, especially at temperatures below about 20 degrees Kelvin, and to avoid the presence of liquid at cryogenic temperatures in the immediate vicinity of the test chamber during operation at temperatures higher than the cryogenic temperature.